Because spheroidal graphite cast irons have excellent mechanical properties and good castability, they are widely used for parts for various machines and automobiles. Among them, suspension parts for automobiles such as suspension arms, steering knuckles, etc. are required to have sufficient static strength and fatigue strength for supporting automobile bodies, as well as enough impact resistance to avoid breakage even under impact due to accidents, etc. Because automobiles are used in cold areas, too, it is important for them to have enough impact resistance at low temperatures, for example, −30° C. Accordingly, spheroidal graphite cast irons for suspension parts are required to have sufficient elongation and toughness such as low-temperature impact strength, etc., in addition to high tensile strength and yield strength. To meet such demands, FCD400, FCD450, etc. defined by JIS G 5502 have conventionally been used as spheroidal graphite cast irons having a ferrite-based matrix structure for high toughness.
To prevent global warming, the reduction of CO2 emission from automobiles has recently been strongly demanded. To this end, automobiles should be provided with improved fuel efficiency, and one of measures therefor is the weight reduction of suspension parts, etc. To reduce the weight of parts while keeping necessary strength, the size reduction and thinning of parts are effective. For this purpose, it may be contemplated to use pearlitic, spheroidal graphite cast irons such as FCD600, FCD700, etc. having higher strength than that of FCD400, FCD450, etc., but FCD600, FCD700, etc. have low toughness, not suitable for suspension parts requiring high impact resistance, because strength and toughness are contradictory properties in spheroidal graphite cast irons. To achieve the weight reduction of suspension parts while keeping their strength and toughness, spheroidal graphite cast irons excellent in both strength and toughness are required.
To obtain spheroidal graphite cast irons having excellent strength and toughness, various proposals have been made conventionally. For example, JP 2001-214233 A proposes a spheroidal graphite cast iron member having as thin-wall portions as 1 cm or less, which is made of a spheroidal graphite cast iron containing 0.5-1% by mass of Cu, and has a surface layer whose matrix has a ferritization ratio of 60% or more, and an inner portion whose matrix is mostly composed of pearlite phases, the surface layer being substantially as thick as 0.05-0.45 mm on the entire as-cast surface, whereby the spheroidal graphite cast iron member has high rigidity and impact resistance. In this spheroidal graphite cast iron member, toughness is obtained from a surface layer as thick as 0.05-0.45 mm with a large proportion of ferrite phases, while strength is obtained from an inner portion composed of a pearlite phase. However, because it is made of conventional pearlitic spheroidal graphite cast iron such as FCD600, FCD700, etc. to have high strength inside, it has low toughness. In addition, when the thickness of a thin ferrite surface layer is reduced by local wear and oxidation, it unlikely keeps toughness necessary for suspension parts.
JP 8-13079 A proposes a method for producing a spheroidal graphite cast iron having ferrite phases in a network pattern along pearlitic crystal grain boundaries for having high strength and toughness, which comprises the steps of heating a spheroidal graphite cast iron comprising by weight 3.0-4.0% of C, 1.5-3.0% of Si, 1.0% or less of Mn, 0.030% or less of P, 0.020% or less of S, less than 1.0% of Cu, and 0.02-0.08% of Mg, the balance being iron, to an austenization temperature T1 (870° C. or higher), holding the spheroidal graphite cast iron at T1 for a predetermined period of time (for example, 2 hours), cooling it to a predetermined temperature T2 (750-850° C.) within a eutectoid transformation temperature range, holding it at T2 for a predetermined period of time (for example, 1 hour), and then cooling it with air to room temperature. However, because the holding temperature T1 for austenization is as high as 870° C. or higher (930° C. in Examples), and because the holding time is as long as 2 hours, austenite crystal grains, which are transformed to pearlite crystal grains by cooling, may be made coarser, resulting in low toughness. Also, because low-strength ferrite phases formed along crystal grain boundaries act as crack-propagating paths, it unlikely has sufficient strength.